Expensive gas substantially increases the price of fuel-efficient used cars, as people become increasingly willing to pay more for a vehicle now to save on fuel costs later. This is what economic theory predicts. For every $1-per-gallon increase in the cost of gas, the price for the most-efficient 25 percent of used cars goes up $2839, on average, finds a new paper at the National Bureau of Economic Research.

But the pattern is far less striking for new cars. Demand for fuel-sipping cars caused by a $1 gas increase drives up the price of the 25 percent most fuel-efficient new cars by only 3 percent. The difference, the authors find, is that the supply of fuel-efficient new cars is much more elastic than the supply of efficient used cars. When gas prices soar manufacturers crank up the production of efficient vehicles. But the supply of used efficient cars, dependent on previous production patterns, is more or less fixed.